This is not very difficult to implement technically, but it does have it's issues. One particular difficulty is the fact that the displaced curve can be self-intersecting (and presumably should not be).

This is not a feature that should be implemented in Blender because there is a much more useful generalization of this idea which could be considered instead. In fact, it is more of a rendering challenge than modelling. It's called "Displacement Mapping", and it is supported by quite a number of the Renderman implementations, including Aqsis. If/when Green finishes Renderman integration, what you describe will be easy to do.

Anyway, the benefit of my suggestion over height maps as they are implemented now is that in my suggestion the "height map" would CREATE vertices where needed. So, instead of subdividing a plane 10 times to have ok results, you just append the image and blender creates vertices where they are needed.